The bigger a dinosaur was, the warmer its blood, a study of the big beasts' fossil remains suggests.

Dinosaurs were long considered to be cold-blooded reptiles.

More recently, some researchers have proposed that the extinct creatures actively regulated their body temperature like mammals.

A study in the journal Plos Biology now suggests this is not the case, but that bigger dinosaurs may have lost heat so slowly that they stayed warm anyway.

Reptiles tend to be cold-blooded ectotherms, whose internal body temperature is dependent on the outside environment. For example, lizards and snakes will sun themselves on rocks in order to heat themselves up.

The question of whether dinosaurs were warm-blooded or cold-blooded just doesn't have a simple answer

Dr Angela Milner, Natural History Museum

Birds and mammals, on the other hand, tend to be warm-blooded endotherms. They can regulate their internal body temperature regardless of external influence.

Their body temperatures tend to be more constant than those of reptiles and higher than the outside environment.

Growth rings

James Gillooly of the University of Florida in Gainesville, US, and colleagues started with an equation showing the relationship between body size, body temperature and growth rate of an animal.

They then applied this equation, which has been shown to be valid across a variety of living species, to dinosaurs.

Gillooly's team used measurements of annual growth rings in the bones of eight dinosaur species to estimate the animals' development rates and body size at full adulthood. This information in turn can be used to calculate the dinosaur body temperature if the equation is re-arranged.

The scientists found the smallest dinosaurs had temperatures of around 25C, close to environmental temperatures and similar to those observed for living reptiles. In other words, they did not actively regulate their internal temperature like mammals and birds.

But as dinosaurs got bigger, they became less efficient at dissipating heat and this helped to keep them warm anyway. This is known as inertial homeothermy.

According to the scientists' equation, the enormous sauropod Apatosaurus - which at 13,000kg was among one of the biggest dinosaurs - had a body temperature of just over 40C.

Huge variety

Most animals cannot tolerate body temperatures of above 45C, so Apatosaurus is both near the upper limit of dinosaur body size and the more general limits for body temperature. This, the authors say, could suggest that the maximum size a dinosaur could grow to was limited by body temperature.

"The question of whether dinosaurs were warm-blooded or cold-blooded just doesn't have a simple answer," said Dr Angela Milner, associate keeper of palaeontology at London's Natural History Museum.

"There's a huge spread of physiological states, from things that were more at the ectothermic end and had no problem keeping warm because they were so large, right through to small meat-eating dinosaurs that were not far short of the endothermic biology seen in birds."

Dr Milner, who is not associated with the authors, pointed out that a group of dinosaurs known as the hadrosaurs apparently switched from being endotherms like mammals and birds in youth to being inertial homeotherms when they were adults.